Simulation of THz Quantum Cascade Lasers

Abstract (Swedish)

Quantum Cascade Lasers (QCLs) have since their invention in 1994 become a crucial source for
mid-infrared and THz radiation. Mid-infrared QCLs have achieved room temperature operation
while THz QCLs have not. As such, simulations are an important tool in order to achieve a
THz QCL operational at room temperature. This work uses a simulation package based on
a nonequilibrium Green’s function model to investigate which Conduction Band Offset (CBO)
should be used for the most accurate results and if there is a difference between samples from
different labs. These CBOs are then used to simulate current the state-of-the-art THz QCLs,
which is done with great success. For example, the thermal degradation of the QCLs was
analyzed and showed that thermal backfilling and thermally activated leakage channels were the
main cause. Also, new QCLs were designed from the information gained. Two of these samples
are predicted according to the simulation package to outperform the current best performing
QCLs.

Popular Abstract

Quantum Cascade Lasers (QCLs) can be designed to emit radiation in the Terahertz (THz) range. For these frequencies, which are faster than electronics and slower than infrared light, there are currently only few devices facing a wide potential of applications. The realization of room temperature operation of THz QCLs would enable practical use in e.g. spectroscopy and public safety. The question is: can computer simulations be the key to
room-temperature THz QCLs?

Lasers are light sources emitting coherent light. Coherent in this context means that the valleys and the peaks of the light wave overlap which enables the light to be focused to a tiny spot. This is in strong contrast to the light emitted by a light bulb, which emits light where the light waves are not in phase with each other and are scattered everywhere.
In the last 60 years, lasers have not only become an integral part of many technologies but also science fiction stories such as Star Trek. Consequently, lasers have become a well-known technology. What is less known is that there exists light not visible for the human eye, for example, infra-red and ultraviolet light and that there exist lasers that emit this “invisible light”.
However, there are parts of the ElectroMagnetic (EM) spectrum not covered by any practical applicable laser. One of these parts is the THz region that is in between the microwave and infrared region of the EM spectrum. Devices that emit THz light exists but have to operate at either unpractically low temperatures or unusably low output powers. No device thus exists that can enable the multitude of possibilities from THz radiation, outside of a controlled lab environment.
These applications are for instance spectroscopy, gas sensing, detection of explosives and drugs as many of the substances has distinct fingerprints in the THz region, and non-invasive medical imaging (replacing cancer-inducing x-rays).
This is why THz QCLs have received a lot of attention recently since the maximum operational temperature has risen a lot in the last 15 years. Currently, the best performing QCLs can operate up to 250 K (~ -23 Co) which facilitates the use of electric coolers such as Peltier coolers. However, in order to be able to utilize THz radiation to its fullest capacity, room-temperature operation is a must.
We have deployed advanced computer models as a means to optimize the current state of the art THz QCLs. It was found that it is necessary to use different material parameters for the same materials depending on which lab manufactured the QCL, indicating that discrepancies exist in the growth procces between different labs. This is of great interest to those who want to simulate QCLs in the future. Furthermore, by using the most fitting material parameters for each manufacturing lab, we managed to simulate the operation of the QCLs with tremendous accuracy.
Even so, the physics learned by simulating already existing samples is of limited use if we cannot apply it to create new, better samples. As such, we looked at the trends from the previous simulations and applied them to create new samples. According to our simulations, these new samples are predicted to operate at even higher temperatures than the current best performing QCL. Of course, the sample must be manufactured and tested in a real world lab environment before any big conclusive statements can be made.
Nevertheless, the results show promise for future developments and point to computer simulations being a key tool to achieve room-temperature operation.

• author: Rindert, Viktor
• supervisor: Andreas Wacker
• organization: Mathematical Physics; Department of Physics
• course: PHYM01 20212
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• language: English
• id: 9069258
• date added to LUP: 2022-01-27 16:08:24
• date last changed: 2022-01-27 16:08:24